The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex, FDD, and Time Division Duplex, TDD, modes.
In an UTRAN and an E-UTRAN, a User Equipment, UE, i.e. a wireless device, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNB, in LTE. A Radio Base Station, RBS, or an access point is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In Wireless Local Area Network, WLAN, systems the wireless device is also denoted as a Station, STA.
LTE uses downlink reference signals transmitted by the eNodeBs. Some of the reference signals are cell specific, which means that, these do not depend/change per user but remain same for all the users in one cell, once configured. A user equipment, UE, receiving the reference signal can measure the quality of neighbor cells for mobility purposes. In LTE, some reference signals are broadcasted in an always-on manner and over the full bandwidth, regardless of the presence or position of UEs in the system. These signals are called cell specific reference signals, CRS, and are easy to measure and yield consistent results, but the static signaling leads to high resource usage, interference, and base station energy consumption.
Handover is an important part of any mobile communications system. In legacy systems, handover is the process of transferring an ongoing connection of a wireless device from one base station (the serving) to another base station (the target), or from one cell to another within the same base station. This is done to accomplish a transparent service or service continuity over a larger area. The handover should happen without any loss of data and preferably with as short interruption as possible. In legacy cell-based systems like LTE, the cell-specific reference signals, CRSs, are used for mobility measurements.
In future communication networks, also referred to as the 5th generation mobile networks, there will be evolvement of the current LTE system to the so called 5G system. One of the main tasks for 5G is to improve throughput and capacity compared to LTE. This is in part to be achieved by increasing the sample rate and bandwidth per carrier. 5G is also focusing on use of higher carrier frequencies i.e., above 5-10 GHz.
Future communications networks are expected to use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. When the antenna is used to increase coverage, handover may be carried out between transmission beams of the serving radio access network node or of the neighbour radio access network nodes. The transmission beam through which the radio access network node is currently communicating with the wireless device is called the serving beam and the transmission beam it will hand over to, or switch to, is called the target beam. The potential target beams for which measurements are needed are called candidate beams.
Applying the principle of continuous transmission of reference signals in all individual transmission beams in such a future cellular communications network may be convenient for wireless device measurements, but it may degrade the performance of the network. For example, continuous transmission of reference signals in all individual transmission beams may consume resources available for data, and generate a lot of interference in neighbouring cells, and higher power consumption of the radio access points.
The LTE-type solution, which implies that detection of any sufficiently strong reference signal causes the wireless device to measure and report, is inefficient in several respects. For example, the wireless device must constantly be exercising the detection algorithm for detecting the reference signals, which entails correlating the received sample sequences against one or more reference sequences. If the network is non-synchronized (or loosely synchronized), the extent of the search window may be large, which causes high computational complexity.
On the network side, the number of reports may be higher compared to LTE, due to more frequent beam updates. The various access nodes must be prepared to receive measurement reports from different wireless devices wherever any reference signal is transmitted in the neighborhood—the exact measurement trigger instances are unknown to the network since it may not be possible to predict exactly which wireless devices can hear which reference signals.
To avoid always-on signaling, one possible approach, that has been discussed for the next generation systems (also referred to as 5G) is that the network turns on reference signals, in a wireless device-specific manner only in relevant candidate beams and in situations when mobility is likely needed (e.g. when signal strength is decreasing and/or load balancing needs to be applied). Then the candidate beams may be selected from a fixed grid of beams. Measurements may then be initiated only when the network obtains an indication that a beam update for the wireless device may be needed, e.g. when decreasing serving beam quality is detected due to wireless device movement, or when the wireless device needs to acquire a serving beam when accessing a new frequency band for the first time. The candidate beams may be transmitted from a single access point or from several access points.
Hence, the network can configure the wireless device (via, e.g., Radio Resource Control, RRC, signaling) to measure and report candidate beam quality, preferably including the list of reference signals to measure. The wireless device thus receives a measurement command indicating the time/frequency resources and sequences of the reference signals to measure, as well as the measurement and reporting configuration. Once the wireless device has performed mobility measurements and reported the results, the network turns the candidate beams off again, i.e. the reference signal transmissions in the candidate beams cease.
Thus, tightly controlled measurement and reporting mechanism avoids the above potential problem of deluge of reports since reports are requested explicitly. However, the signaling overhead associated with initiating the individual measurement sessions may be a drawback in some systems and deployments. Thus, there a need for improved methods of initiating measurements.